The present invention relates to a method and apparatus for measuring photoelectric conversion characteristics and, more particularly, to a method and apparatus for measuring the photoelectric conversion characteristics of a photoelectric conversion device such as a solar cell, photodiode, photosensor, or electrophotographic photosensitive body and, especially, a stacked photoelectric conversion device.
In a stacked photoelectric conversion device in which a plurality of photoelectric conversion elements with different spectral responses are stacked, long-wavelength light that cannot be completely absorbed by the upper photoelectric conversion element on the light incident side is absorbed by the lower photoelectric conversion element, thereby increasing the output or sensitivity. Hence, such stacked photoelectric conversion devices have been extensively developed.
It is very important to accurately measure the output characteristics of a stacked photoelectric conversion device due to the following reasons.
For example, in manufacturing and delivering stacked photoelectric conversion devices whose maximum power is important, a photoelectric conversion device whose maximum power is less than a rated value is determined as a defective product by inspection. However, the maximum power of a photoelectric conversion device to be delivered cannot be guaranteed unless the output can be accurately measured. In addition, if an output measurement error is large, and the measurement error changes depending on the state of the measuring apparatus, the inspection threshold value varies even for photoelectric conversion devices with the same quality, resulting in unstable manufacturing yield. Furthermore, if the inspection threshold value contains a measurement error value to guarantee the quality of photoelectric conversion devices to be delivered, the manufacturing yield inevitably decreases.
If the output of a stacked photoelectric conversion device cannot be accurately predicted, no predicted system characteristic can be obtained or the system efficiency degrades in building a system using the stacked photoelectric conversion device. When the stacked photoelectric conversion device is a solar cell, it considerably affects, e.g., the guaranteed maximum power of the solar cell, manufacturing yield, power generation prediction of a power generation system, and system efficiency.
However, it is very difficult to accurately measure the output characteristics of a stacked photoelectric conversion device. The main reason for this is that the output characteristics of the stacked photoelectric conversion device largely change depending on the spectrum of irradiation light. For example, a double-type solar cell (to be referred to as a xe2x80x9cdouble cellxe2x80x9d hereinafter) in which two semiconductor junctions are stacked and connected in series will be described in detail. The upper semiconductor junction on the light incident side is called a top cell, and the lower semiconductor junction is called a bottom cell. The short-circuit current of each cell changes depending on the spectrum of irradiation light because the cells have different spectral responses. As a result, the short-circuit current, fill factor, and open-circuit voltage of the entire double cell change, and the output characteristics of the double cell largely change.
To the contrary, in a single-type cell (to be referred to as a xe2x80x9csingle cellxe2x80x9d hereinafter) having a single semiconductor junction, only the short-circuit current changes depending on the spectrum of irradiation light, and the fill factor and open-circuit voltage are rarely affected. For this reason, when the spectrum dependence of the short-circuit current is corrected, the output characteristics can be almost accurately measured.
Generally, to accurately measure the output characteristics of a photoelectric conversion device, test conditions such as the intensity and spectrum of irradiation light and the temperature of the photoelectric conversion device must be defined. For, e.g., a solar cell, the test conditions are defined as standard test conditions as follows.
Temperature of solar cell: 25xc2x0 C.
Spectrum of irradiation light: standard sunlight (The spectrum of standard sunlight is defined by JIS C 8911)
Irradiance of irradiation light: 1,000 W/m2 
However, of these standard test conditions, the spectrum of standard sunlight can hardly be obtained even when outdoor sunlight is used. This is because the standard sunlight is obtained only under limited meteorological conditions. It is impossible to obtain the spectrum of standard sunlight using a pseudo sunlight source indoors.
For a single cell, pseudo sunlight sources (solar simulators) are classified into ranks A, B, and C sequentially from one close to the standard sunlight on the basis of the spectrum, variation (to be referred to as a xe2x80x9cpositional variationxe2x80x9d hereinafter) in irradiance depending on the position, and time variation ratio. This ranking is described by JIS C 8912 and JIS C 8933. Using a solar simulator of rank A or B and a secondary reference solar cell having a spectral response similar to that of a solar cell to be measured, the irradiance of the solar simulator is set, thereby correcting an error due to a shift in spectrum. This measuring method is described by JIS C 8913 and JIS C 8934.
The above measuring method is possible for a single cell for which the spectrum affects almost only the short-circuit current. However, in a stacked solar cell, the spectrum affects not only the short-circuit current but also the fill factor and open-circuit voltage, as described above, and the output characteristics cannot be accurately measured by the above measuring method. Hence, the stacked solar cell is excluded from the above-described JIS.
The following technique has been proposed as a method of accurately measuring the output characteristics of a stacked solar cell.
The spectrum of a solar simulator used to measure a stacked solar cell is made adjustable and adjusted to obtain short-circuit current and fill factor values that the stacked solar cell probably generates under standard sunlight, thereby accurately measuring the output characteristics (this technique will be referred to as a xe2x80x9cmulti-source methodxe2x80x9d hereinafter) (T. Glatfelter and J. Burdick, 19th IEEE Photovoltaic Specialists Conference, 1987, pp. 1187-1193).
That is, each of a plurality of semiconductor junctions of a stacked solar cell is defined as a component cell. Let In.ref (n is the number of each component cell) be the short-circuit current generated by each component cell in the stacked solar cell under standard sunlight and In.test be the short-circuit current generated under a solar simulator. Then, when the spectrum of the solar simulator is adjusted to satisfy
In.test=In.refxe2x80x83xe2x80x83(1) 
for each component cell, the short-circuit current and fill factor of the stacked solar cell match the values under the standard sunlight.
The above measurement technique assumes use of a solar simulator having an adjustable spectrum. In the above-described reference, to adjust the short-circuit current of each component cell, light components from three light sources: one xenon (Xe) lamp and two halogen lamps are separated into three wavelength bands and then synthesized. By adjusting the irradiances of the three light sources, the intensities of light components in the three wavelength bands are controlled, thereby adjusting the spectrum of the synthesized light.
The solar simulator with variable spectrum is possible for a small irradiation area of 400 cm2 or less. However, due to the following reasons, it is very difficult to manufacture a solar simulator having an area more than 400 cm2.
(i) Since a plurality of light components having different spectra are synthesized, the positional variations in spectrum of the synthesized light and in irradiance become large. The larger the irradiation area is, the more serious these variations become.
(ii) Since the spectrum of partial light from the light source is used, the light intensity tends to be short. When the irradiation area becomes large, it is hard to obtain an irradiance of the standard test condition of 1,000 W/m2.
(iii) The structure becomes complex, and the manufacturing cost largely increases as compared to a normal solar simulator using a single light source.
(iv) Adjustment of the solar simulator with variable spectrum is cumbersome, and its control requires skill.
The multi-source method can accurately measure the output characteristics of a stacked solar cell. However, due to the above reasons, the light-receiving area of the solar cell to be measured is limited to the minimal area of laboratory level, and it is hard to measure a cell, module, or array having an area more than 400 cm2. Even when a measuring apparatus for the multi-source method can be manufactured, the cost is very high.
In addition, the multi-source method cannot be applied to outdoor measurement using sunlight. However, to, e.g., check the output of a solar power generation system, it is necessary to measure a stacked photoelectric conversion device installed outdoors using sunlight. In measuring the output characteristics of the stacked photoelectric conversion device, no accurate measurement result can be obtained unless a change in output characteristics of the stacked photoelectric conversion device due to a change in sunlight spectrum is corrected. This correction is a problem kept unsolved. Furthermore, in predicting the power generation amount of a stacked photoelectric conversion device outdoors in accordance with the region or season, it is important for accurate power generation amount prediction to take into consideration the change in output characteristics of the stacked photoelectric conversion device due to the change in sunlight spectrum.
However, the change in sunlight spectrum is not uniform. Additionally, the change in sunlight spectrum is represented using a number of indices including the air mass, turbidity, and precipitable water as main indices that have large influence. The manner the output characteristics of the stacked photoelectric conversion device change differs for each index. Hence, it is very difficult to calculate or correct the change in output characteristics of the stacked photoelectric conversion device due to the change in sunlight spectrum.
The present invention has been made to solve the above-described problems individually or altogether, and has as its object to measure the photoelectric conversion characteristics of a wide-area photoelectric conversion device such as a module or array independently of the area of a stacked photoelectric conversion device to be measured.
It is another object of the present invention to accurately measure the photoelectric conversion characteristics of a stacked photoelectric conversion device using an inexpensive measuring system.
In order to achieve the above objects, according to a preferred aspect of the present invention, a method of measuring photoelectric conversion characteristics of a stacked photoelectric conversion device having a structure in which a plurality of semiconductor junctions are stacked, comprising the steps of measuring output characteristics of the photoelectric conversion device under irradiation light having a plurality of different spectral states, estimating a shift, from a standard test condition, of a short-circuit current of a component cell formed by each of the plurality of semiconductor junctions of the photoelectric conversion device, and obtaining the photoelectric conversion characteristics of the photoelectric conversion device in the standard test condition by comparing the measured photoelectric conversion characteristics with the estimated shift of the short-circuit current from the standard test condition is disclosed.
It is still another object of the present invention to measure the photoelectric conversion characteristics of a stacked photoelectric conversion device both indoors and outdoors.
In order to achieve the above object, according to a preferred aspect of the present invention, the method wherein the plurality of different spectral states are realized by using pseudo sunlight as the irradiation light and changing or exchanging some components of an optical system for irradiation is disclosed.
It is still another object of the present invention to quantify the spectrum dependence of the photoelectric conversion characteristics of a stacked photoelectric conversion device.
In order to achieve the above object, according to a preferred aspect of the present invention, a method of quantifying spectrum dependence comprising the step of representing a change in spectrum of the irradiation light as an index on the basis of a change in shift of the short-circuit current of the component cell from the standard test condition and quantifying the spectrum dependence of the photoelectric conversion characteristics of the photoelectric conversion device is disclosed.
It is still another object of the present invention to predict the output characteristics of a stacked photoelectric conversion device.
In order to achieve the above object, according to a preferred aspect of the present invention, a method of predicting photoelectric conversion characteristics of a photoelectric conversion device comprising the step of predicting the photoelectric conversion characteristics of the photoelectric conversion device in an arbitrary spectral state on the basis of a change in shift of the short-circuit current of the component cell from the standard test condition is disclosed.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.